The present invention relates to a rotor assembly for a screw pump which compresses a gas and is used for supercharging of an engine or the like, and more particularly to a means for fixing a shaft and a rotor main body in the rotor assembly.
For example, in Japanese Published Examined Utility Model Registration Application No. 21192/1989, there is known a rotor assembly of a pump for handling a gas, in which a guide portion and a serrated portion are sequentially provided in a shaft in the longitudinal direction thereof, the shaft is pressingly fitted or press-fitted into a central bore in a rotor main body, and a pin which penetrates the rotor main body and the shaft in the diametrical direction is press-fitted at a substantially central position in the longitudinal direction of the rotor assembly, thereby fixing the rotor main body and the shaft together.
In this kind of rotor assembly, it is normal practice to make the shaft of a steel, and to make the rotor main body of an aluminum alloy so that the rotor main body can be made light in weight and that the coefficient of thermal expansion becomes equal to that of a housing of the pump. However, the rotor assembly is heated by the compression heat of the gas, and the coefficient of thermal expansion of the steel of 11.times.10.sup.-6 is largely different from that of the aluminum alloy of 20 to 23.times.10.sup.-6, resulting in a large difference in the amount of expansion and contraction due to the changes in temperature. It follows that, if the pin is located in the shaft and the rotor main body at a position where an axial displacement relative to each other is likely to occur and the displacement is large, there will occur an excessive bearing pressure or surface pressure on the rotor main body at the portion where the pin is press-fitted into a pin hole in the rotor main body, accompanied by a fear that the pin hole is so enlarged that the interference fit condition can no longer be maintained.